1.  Jordan R. Dodd, ed.  1993.  North Carolina Marriages: Early to 1850.  Liahona Research, Orem, UT (Broderbund CD-229):

Johnston, John

Clark, Sarah

16 Dec 1797

Orange Co.

2.  Census Index:  U.S. Selected Counties, 1800 (Broderbund CD-312):  there were no JOHNSTONs in Orange Co., but there were JOHNSONs.  There was no John JOHNSON with a head-of-household the right age to be our subject.  The newlyweds may have been living with his parents, so I checked all the JOHNSON households containing a younger couple the right ages to be John & Sarah plus a small son.  The only one that filled the bill was this one, which also means John's father was named John.  Please see my WARNING regarding use of this database (the numerical data have the genders switched). 

1800

Johnson, John

NC

Orange Co.

p. 576

01101-1010100

These data should read:

10101-01101-00

which indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	9 or under	1790-1800	= John III (b. 1798)
1 male	16-25	1774-1784	= John Jr. (b. 1779)
1 male	45 or over	in or bef. 1755	= John, father of John Jr.
1 female	10-15	1784-1790	= Sister of John Jr.
1 female	16-25	1774-1784	= Wife of John Jr.
1 female	45 or over	in or bef. 1755	= Mother of John Jr.

I also checked the CLARK households, on the off chance that they were living with Sarah's parents.  There were two possibilities (numbers corrected for the gender switch):

1800	Clark, Thomas	NC	Orange Co.	p. 558	10110-00311-01
1800	Clark, William	NC	Orange Co.	p. 557	10201-00201-00

3.  Census Index:  U.S. Selected Counties, 1810 (Broderbund CD-313):

1810

Johnston, John

NC

Orange Co.

p. 113

4.  Census Index:  U.S. Selected Counties, 1820 (Broderbund CD-314):

1820

Johnston, John

NC

Orange Co.

p. 328

5.  Census Index:  U.S. Selected Counties, 1830 (Broderbund CD-315):

1830

Johnston, John

NC

Orange Co.

p. 277

p. 298

6.  Census Index:  U.S. Selected Counties, 1840 (Broderbund CD-316):

1840	Johnson, John	AR	Madison Co.	Richland Twp.	p. 25
1841	Johnson, John	AR	Madison Co.

7. 1850 Census (online at the Madison County ARGenWeb site):  Kings River Twp., Madison Co., AR, 398/398 :

William K. Johnson	41	blacksmith	NC
Doritha	38		TN
[eight children of William & Doritha]
John Johnson	72		NC

Living next door to James & Mary Ann (JOHNSTON) BASHAM — Mary Ann is John's granddaughter, through his son, Samuel. 

8.  Broderbund.  World Family Trees.  Vol. 22, Pedigree No.  11.

9.  Lisa Mashburn's Johnston Family Tree. [link died]

10.  LDS.  Family Search: Internet Genealogy Service: Ancestral File (AF).

11.  Email from Larry Kuttner.